Optical disks of various kinds are made of circular plates with reflecting coating material which have concentric circular or spiral tracks on which the data is written in the form of spots of changes of some optical property of the disk coating media. These changes are typically spots of lower reflection than the non data-containing parts of the disk. These data spots are spread along a spiral or circular path on the disk. The disk is divided into sectors, and each loop of this spiral path that lies within a sector is called a track.
Data is read from the optical disk by an electro optical head which projects a high brightness focused laser beam on the rotating disk. The reflected light from the disk is detected and transduced into an electrical signal. At any point in time the electro-optical head reads one data spot. Rotation of the disk enables reading a track spot by spot. In order to read data from inner or outer tracks, the electro-optical head is moved to the desired track position by a motor. The reading of the data is done serially along a spiral path.
Data transfer rates of commercially available CD-ROMs (Compact Disk--Read Only Memory) is about 157 KByte/sec. CD-ROMs memory storage capacity is limited to about 640 MBytes (see, e.g., the publication E-93176 PCO-0032, 4/87 to Philips and DuPont Optical). The limit of the memory storage capacity is a function of the wavelength and the numerical aperture of the objective lens. One known way to increase memory capacity in all sorts of optical memory disks, such as CD-ROMs, WORM (Write Once Read Many) disks, and magneto-optical disks, is to decrease the wavelength of the light emitted by the diode laser which illuminates the data tracks of the optical memory disk. Wavelength reduction results in smaller data spots on the disk and thus leads to higher resolutions and enhanced data densities. Current CD-ROMs employ wavelength of 780 nanometers (nm).
The advent of new diode lasers which emit, for example, blue light (around 481 nm) raises the potential of enhancing optical disk data density. However, blue light diode lasers still emit low energies which are not (yet) suitable for the process of reading/writing on an optical disk. Moreover, they are expensive and have very short operating life.
Another way to achieve blue radiation is by frequency doubling of infrared laser by non-linear optical material. However, such systems have very low efficiency and are not yet commercially available ("Compact Blue Laser Devices Based on Non-Linear Frequency Upconversion". W. P. Risk and W. lenth, IBM Research Division, SPIE Vol. 7104, 1989).
In an article by Demetri Psaltis, "Parallel Optical Memories", BYTE Magazine, September 1992, pp. 179, parallel access to optical disks is mentioned by illuminating the disk with a broad optical beam instead of a tightly focused spot. It is further mentioned that an increase in storage density is required and this can be achieved by development of shorter wavelength semiconductor lasers because the minimum area required for storing a pixel is equal to a wavelength squared.
In another article by D. C. Kowalski et at., "Multichannel digital optical disk memory system", Optical Engineering, Vol. 22, No. 4, July/August 1983, pp. 464, there are described write and read optical head of optical disk systems which employ gas lasers for writing and reading data. The writing laser is a high power argon laser and the reading is done by a lower power HeNe laser. Both lasers are expensive and bulky when compared to solid state diode lasers used in commercial devices.
Other prior art systems are described in an article by Demetri Psaltis, "Optical memory disks in optical information processing", Applied Optics, Vol. 29, No. 14, May 10, 1990, and in U.S. Pat. No. 5,111,445 to Psaltis et at.
In U.S. patent application Ser. No. 043,254 filed on Apr. 6, 1993 (corresponding to EP 93105995.0), the description of which is incorporated herein by reference, an optical reading mechanism of optical disks is described which enables readout of many tracks in parallel by employing a CCD/TDI detector matrix, a laser diode illumination system, and an image processor apparatus for tracking and detecting.
So far, prior art systems have employed laser diodes for illuminating optical disks, or has sought to improve the performance of optical disk systems by providing more sophisticated lasers, as described above. The prior art systems, however, present severe drawbacks, particularly because of the high cost of providing improved laser light for CD illumination purposes. It is therefore clear that it would be highly desirable to be able to provide improved systems which do not require such expensive or low efficiency laser sources.